This invention relates generally to a small-size type-printing serial printer and more particularly to a printer capable of multicolor printing operation. Known serial printers capable of two-color printing operation include large-size daisy wheel printers, badminton printers, ball printers and the like using two-color ink ribbons. Small-size printers for use on electronic desktop calculators are available in limited types. One known such small-sized printer is a serial printer construction proposed in Japanese Laid-Open Patent Publication No. 56-28885, which is capable of only single-color printing. This type of serial printer comprises two type wheels each supporting a series of type around its outer peripheral edge and movable at incremental steps along a guide shaft to print character by character across the sheet. In a standby position, one of the two type wheels for printing characters in a first color is located in a lowest character or digit position, and the other type wheel for printing characters in a second color is located in a highest character or digit position. For printing characters in the first color, the first-color printing type wheel is shifted from the lowest character position successively toward the highest character position. Conversely, for printing characters in the second color, the second-color printing type wheel is shifted from the highest character position successively toward the lowest character position.
Where a printer of the above type is incorporated in an electronic desktop calculator, the number of digits usually appearing in the printer across the line is far smaller than the maximum number of digits that can be printed. When characters are to be printed in the first color, the first-color printing type wheel moves stepwise from the standby or lowest character position until all necessary digits are printed, whereupon the type wheel returns to the standby position. Therefore, the first-color printing type wheel can print characters speedily. For printing digits in the second color, however, the second-color printing type wheel has to traverse a blank space from the standby or highest character position before it can print necessary digits down to the lowest character position. Therefore, the second-color printing type wheel is required to move across all of the digit positions before it returns to its standby position, a disadvantage which greatly reduces the printing speed. The operator who uses a calculator incorporating this type of printer tends to become irritated because of the slow printing speed. Furthermore, the prior art printer is unable to effect multicolor printing operation using three different colors or more.
What is needed is a serial printer capable of printing in a plurality of colors with a speed substantially equal to that of a monochrome serial printer.
The present invention has been made in view of the above prior art difficulties.